The invention relates in general to semiconductor devices and in particular to a new and useful semiconductor device for converting light into electric energy which includes at least one layer of amorphous silicon.
In semiconductor devices of the mentioned kind, amorphous silicon (a-Si) has proved substantially less expensive in manufacture than mono-or polycrystalline silicon (see D. E. Carlson and C. R. Wronski, Appl. Phys. Lett., 28, 671 (1976)). Because of its considerably higher optical absorption, amorphous silicon is needed only in layers having a thickness of about 1 micron, as compared to 50 to 100 microns with crystalline silicon (see German OS 2,632,987, page 7). Also, a-Si may be deposited directly from the gaseous phase on relatively inexpensive substrates and, in contradistinction to crystalline silicon, does not require any further expensive treatment of the material. The manufacture of such semiconductor devices is comprehensively described in W. E. Spear et al, Appl. Phys. Lett., 28,105 (1976) and in U.S. Pat. No. 4,196,438.
All known a-Si cells with a p-i-n or p-n structure have substantially less degradation than those with m-i-s structures; however they show a drop in collection efficiency upon irradiation with blue-violet light (see FIG. 1). This fact, pointing to a poor conversion of the intense blue part of solar radiation, contributes to the low efficiency of prior art a-Si cells, as compared to crystalline silicon cells.
The poor yield in blue light will be understood while considering that in a-Si, the optical depth of penetration of the blue part of the solar spectrum is smaller than 0.1 micron. The blue light is absorbed substantially in the near-surface a-Si contact layer which is mostly p.sup.+ -doped and has a thickness of about 500 angstroms. The charge carriers produced there have a shortened life time mostly because of the heavy doping, therefore, they can hardly flow to the diode junction and contribute to the photocurrent.